In Situ Generation of Ultrathin MoS2 Nanosheets in Carbon Matrix for High Energy Density Photo‐Responsive Supercapacitors

Abstract Stimuli‐responsive supercapacitors have attracted broad interest in constructing self‐powered smart devices. However, due to the demand for high cyclic stability, supercapacitors usually utilize stable or inert electrode materials, which are difficult to exhibit dynamic or stimuli‐responsive behavior. Herein, this issue is addressed by designing a MoS2@carbon core‐shell structure with ultrathin MoS2 nanosheets incorporated in the carbon matrix. In the three‐electrode system, MoS2@carbon delivers a specific capacitance of 1302 F g−1 at a current density of 1.0 A g−1 and shows a 90% capacitance retention after 10 000 charging‐discharging cycles. The MoS2@carbon‐based asymmetric supercapacitor displays an energy density of 75.1 Wh kg−1 at the power density of 900 W kg−1. Because the photo‐generated electrons can efficiently migrate from MoS2 nanosheets to the carbon matrix, the assembled photo‐responsive supercapacitor can answer the stimulation of ultraviolet‐visible‐near infrared illumination by increasing the capacitance. Particularly, under the stimulation of UV light (365 nm, 0.08 W cm−2), the device exhibits a ≈4.50% (≈13.9 F g−1) increase in capacitance after each charging‐discharging cycle. The study provides a guideline for designing multi‐functional supercapacitors that serve as both the energy supplier and the photo‐detector.

was obtained after centrifugation, washing and drying in vacuum at 60 o C.

Preparation of MoS 2 @carbon
MoS 2 @carbon was obtained by a two-step thermolysis process. MoS 4 2--BP was firstly dried at 100 ℃ for 1 h under the protection of 5% H 2 /95%N 2 atmosphere, and then heated to 500℃ at a heating rate of 5 ℃/min. This temperature was maintained for 1 h. Then, the sample was heated to 950 ℃ at a heating rate of 5 ℃/min under the protection of Ar atmosphere. After 0.5 h reaction, the system was naturally cooled to room temperature. Control samples of CBP and b-MoS 2 were prepared by sintering BP and (NH 4 ) 2 MoS 4 under the same condition.

Characterization
The crystalline structure of the samples was characterized by X-ray diffraction (Bruker-AXS D8-A25) using Cu Kα radiation (λ=0.15141 nm). The elemental composition of the samples were analyzed by using PHI Quantum-2000 photo-electron spectrometer (Al Kα with SEX300D/300DUVA Xenon lamp light source assembled with 365, 450, 550, 650, 808 nm filters was adopted in the photo-response characterization.

Calculation of the average size of MoS 2 @carbon, CBP, and BP
The average sizes of MoS 2 @carbon, CBP, and BP were calculated from TEM images by Digital Micrograph (statistical magnitude is 100) with Equation S(1): where D is the average size of MoS 2 @carbon, CBP, and BP; d i is the particle size measured from TEM image; i is the sequence number of selected particles.
The standard deviations of MoS 2 @carbon, CBP, and BP were calculated by Equation S(2): where s is the standard deviation of MoS 2 @carbon, CBP, and BP; D is the average size of MoS 2 @carbon, CBP, and BP; d i is the particle size measured by TEM image; i is the sequence number of selected particles.

Electrochemical measurement
Electrochemical workstation (CHI 760E) was applied for the electrochemical measurements. In the typical three-electrode system, a reference electrode Ag/AgCl, a counter electrode Pt foil, and a working electrode glassy carbon were used. All potentials of the three-electrode system were referred to RHE.
The symmetric and asymmetric supercapacitors are constructed using the two-electrode system. The gel electrolyte was prepared by dissolving 5.0 g PVA in 30 mL of 1 M H 2 SO 4 aqueous solution. To construct the symmetric supercapacitor, the MoS 2 @carbon slurry was coated on carbon papers to act as both positive and negative electrodes (mass loading: 2 mg cm -2 ). When fabricating the photo-responsive device PRSC, MoS 2 @carbon slurry was coated on the ITO glasses.
In the asymmetric supercapacitor, MoS 2 @carbon slurry was coated on the carbon paper to form the positive electrode (mass loading: 2 mg cm -2 ), and activated carbon slurry was coated on the carbon paper to serve as the negative electrode. The mass of active carbon slurry is determined by Equation S(4): Where m (g) is the mass of electroactive materials, C (F g -1 ) is the specific capacitance, ΔV (V) is the voltage range.
The specific capacitance (C, F g -1 ) of the samples was determined by the discharging portion of the GCD curves, and calculated by Equation S(5): Where, I (A) is the discharging current, Δt (s) is the discharging time, and ΔV (V) is the voltage. In the three-electrode system, m (g) is the mass of the active material coated on the working electrode. In the two-electrode system (symmetric and asymmetric supercapacitors), m (g) represents the mass of the active material in the positive electrode and negative electrode.
The energy density (E, Wh kg -1 ) and power density (P, W kg -1 ) of the symmetric and asymmetric supercapacitors were estimated by Equation S(6) and (7), respectively: and Where, C (F g -1 ) represents the specific capacitance of the symmetric and asymmetric supercapacitors, ΔV (V) represents the voltage, Δt (s) represents the discharging time.
The CV curves (three-electrode system) of MoS 2 @carbon at various scanning rates were used to determine the charge storage mechanism. Peak currents (i, A) were obtained at different scanning rates (v, mV/s) from the CV curves. The charge storage mechanism of the electrode materials can be predicted by calculating b using Equation S(8) and (9): [2] i av b (8) Log i b Log v+Log a (9) where a, b are constants and b is generally in the range of 0.5-1. b = 0.5 indicates the diffusion-controlled process, while b=1.0 indicates the surface-controlled process. If b is in the range of 0.5-1, the electrode material exhibits both diffusion-controlled process and the surface-controlled process.
The contribution of the surface-controlled capacitance and diffusion-controlled capacitance to the total capacitance was calculated by Equation S(10) and (11): [3] I V k 1 v+k 2 v 1/2 (10) Where, I V is the current at a fixed voltage, v is the scanning rate, k 1 and k 2 are constants.
Towards a specific voltage, k 1 is obtained by linearly fitting of i V / v 1/2 and v 1/2 . Different specific voltages correspond to different fitted k 1 . Simultaneously, k 1 v and k 2 v 1/2 correspond to the current contributions from the surface capacitive effect and the diffusion-controlled intercalation process, respectively. The surface-controlled capacitance also reflects the rate performance of the electrode materials as well as the surface charge storage capability.

Photo-response measurement of MoS 2 @carbon
The light-triggered capacitance evolution of MoS 2 @carbon was first investigated in the three-electrode system, in which Ag/AgCl, Pt foil, glassy carbon, and 1 M H 2 SO 4 were used as the reference electrode, counter electrode, working electrode, and electrolyte, respectively.         respectively. In comparison, the specific capacitances of CBP and b-MoS 2 are 462 and 23.3 F g -1 , respectively, at a current density of 1 A g -1 . In the Nyquist plots, the slope of MoS 2 @carbon is higher than those of CBP and b-MoS 2 at low frequencies, indicating that MoS 2 @carbon shows better capacitive performance. From Figure S11a, the values of b are calculated to be 0.90 for the anode and 0.84 for the cathode, implying that the surface-controlled process is dominant for the charge storage.
Moreover, 74.62 % and 58.94 % of the total capacitances are contributed by the surfacecontrolled process at 50 and 5 mV s -1 , respectively (Figure S11b-d). This result implies that the contribution of diffusion-controlled capacitance drops rapidly with the increasing scanning rate.